Titanium oxide particles exhibit intense oxidation upon absorption of an ultraviolet ray and are recently used as photocatalysts in a variety of applications such as applications (1) to (5):
(1) air purification by removing environmental pollutants, such as nitrogen oxides (NOx) and sulfur oxides (SOx) which are emitted typically in automobile exhaust gases;
(2) deodorization by removing malodor-causative substances such as ammonia, acetaldehyde, hydrogen sulfide, and methyl mercaptan;
(3) water purification by decomposing and removing organochlorine compounds such as tetrachloroethylene and trihalomethanes;
(4) antimicrobial applications by killing microorganisms and further decomposing the dead microorganisms; and
(5) antifouling applications by decomposing oils to prevent soiling caused by the attachment of sand and dirt to the oils.
Titanium oxide photocatalysts are roughly classified into one used as being suspended in solutions; and one used as supported on substrates (carriers). In general, the former has higher activities as compared with the latter, because such catalysts generally have a photocatalytic activity proportional to the size of surface area. However, the latter is more often employed as compared with the former from the viewpoint of practicality. In the case of the latter, the titanium oxide photocatalyst may be used mainly by highly dispersing the titanium oxide photocatalyst using a dispersing agent, and bringing the highly dispersed titanium oxide photocatalyst into intimate contact with a substrate using a binder component.
Known examples of the dispersing agent to be used include polymeric organic compounds such as polycarboxylates, salts of naphthalenesulfonic acid/formalin condensates, and polyethylene glycols; and inorganic compounds such as hexametaphosphates, pyrophosphates, phosphates, and oxalates (Patent Literature (PTL) 1 to 3). The dispersing agent, when used, effectively allows the titanium oxide photocatalyst to be highly dispersed, to have a larger specific surface area, and to have a higher photocatalytic activity. However, the dispersing agent, when present on or over a surface of the titanium oxide photocatalyst, preferentially receives oxidation in a portion overlying the surface of the titanium oxide photocatalyst, and this reduces the apparent photocatalytic effect of the photocatalyst. In particular, assume that the titanium oxide photocatalyst employs an inorganic compound as the dispersing agent. Disadvantageously, the titanium oxide photocatalyst in this case fails to develop the inherent photocatalyst effect, because such inorganic compound resists decomposition by the photocatalysis of the titanium oxide photocatalyst. In addition, the titanium oxide photocatalyst employing the inorganic compound as the dispersing agent has excellent initial dispersibility, but hardly maintains a high dispersion state over a long time, and has poor dispersion stability.
In contrast, when the titanium oxide photocatalyst employs a polymeric organic compound as the dispersing agent, the dispersing agent present in the surface of the titanium oxide photocatalyst is preferentially decomposed. Assume that the polymeric organic compound is used in such a sufficient amount as to highly disperse the titanium oxide photocatalyst. Disadvantageously, it takes a long waiting time for the titanium oxide photocatalyst in this case to develop the inherent photocatalyst effect. Assume that the titanium oxide photocatalyst employs a polymeric organic compound having a low molecular weight. Unfortunately, the polymeric organic compound hardly gives effects as the dispersing agent, although it can contribute to a shorter waiting time for the titanium oxide photocatalyst to develop the photocatalytic activity.
PTL 4 describes a binder component employing titanium peroxide in combination with a polysaccharide such as chitosan or cellulose. Disadvantageously, however, the polysaccharide such as chitosan or cellulose is insoluble in water and should be dissolved in a large amount of an aqueous hydrogen peroxide solution at a high temperature so as to prepare a binder liquid. In addition, the resulting coating liquid becomes acidic and is limited in applications.